CA2474367A1

CA2474367A1 - Electrolytic jet plasma process and apparatus for cleaning, case hardening, coating and anodizing

Info

Publication number: CA2474367A1
Application number: CA002474367A
Authority: CA
Inventors: Jingzeng Zhang
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-07-26
Filing date: 2004-07-26
Publication date: 2006-01-26

Abstract

This invention relates to the plasma process in electrolytic or combinative electrolyte/gas/suspension environments for cleaning, case hardening, coating and anodizing of ferrous and non-ferrous metals and their alloys as well as composites. The plasma process carried on the material surfaces is established by arranging the materials to form the cathode or anode of an electrolytic cell circuit in which the counter-electrode is the electrolytic jet head and the jetted electrolyte is the electrically conductive medium between the electrodes biased by an AC, DC or pulse DC voltage in excess of 100 V. The electrolyte used is an aqueous solution impinged with air/gas into electrolytic flow or incorporated with powder additives into the solution at various percentages.
Without requirement of a large power, the process can be adapted for cleaning, case hardening (i.e., carburizing, carbonitriding, nitrocarburizing and nitriding) and coating (i.e., metallic, ceramic, polymeric and composite) of localized and large conductive material surfaces when the surfaces are as the cathode and for anodizing of lightweight (e.g., Mg, A1 and Ti) metals and alloys when the materials are as the anode.

Description

Electrolytic Jet Plasma Process and Apparatus for Cleaning, Case Hardening, Coating and Anodizing AREA OF TECHNOLOGY
This invention relates to the plasma process in electrolytic or combinative electrolyte/gas/suspension environments for cleaning, case hardening, coating and anodizing of ferrous and non-ferrous metals and their alloys as well as composites. The invention may be used as an environmental-friendly and co st-effective surface engineering process for applications in automotive, aerospace, manufacturing, military, and biomedical industries.
Metallic surfaces may have contamination such as rust, oil, pigmented drawing compound, chips, and cutting fluid, and others which usually needs to be removed before coating. Traditional methods of cleaning metallic surfaces including acid pickling, abrasive blasting, wet or dry tumbling, blushing, and salt-bath descaling which usually have cost or environmental problems. The process in this invention can be used to clean the metallic surfaces in an environmental-friendly manner.
Many large engineering components and deforming dies/tools need whole or localized surface hardening such as carburizing, carbonitriding, nitrocarburizing and nitriding, however they cannot fit in a chamber for the treatment due to the limited chamber size.
Thus, there is a demand of a case hardening process which does not need a closed chamber. The process in this invention can be used to case hardening with operation in an open-periphery container.
Numerous items and components manufactured from ferrous and non-ferrous metals and alloys suffer from wear and corrosion problems. Chromium (Cr) and cadmium (Cd) coatings are widely used for particular corrosion protections. However, the coatings and coating process cause environmental hazards. The process in this invention may be used as an alternative process for replacing environmental-unfriendly techniques presently used.
Because of their physical and mechanical properties and the process used to manufacturing items of complex and/or large configuration, lightweight metals and alloys (both wrought and casting) are increasingly used in, for example, automotive and aerospace industries. Protective coatings on these material surfaces with enhanced wear and corrosion resistances are urgently needed. The process in this invention possesses a unique advantage in producing surface coatings with high wear and corrosion resistance on lightweight metallic materials by use of a reasonably low power supply.
PRIOR ART
U.S. Pat. No. 5,981,084 describes an electrolytic process for cleaning an electrically conducting surface in which an electrolytic cell includes a cathode comprising the surface of the workpiece and an inert or erosive anode with a perforated flat or curved surface.
The gap distance between anodic and catholic electrodes is at the range of 5-20 mm, otherwise the micro-arc or electro-plasma cannot be generated (X. Nie, et al, Electrolytic Plasma Processing for Cleaning and Metal-coating of Steel Surface, Surface 8c Coating Technology, vol 150, pp. 246-256, 2002). Such a configuration of the electrolytic cell, however, may not allow cleaning the workpiece with a complex shape. U.S. Pat.
No.
6,585,675 and 6,585,875 describe a modified electrolytic process wherein a particularly designed electrolytic cell provides more uniform foam by vaporizing electrolyte. The process is specifically suitable for cleaning wires and rods. Because of the closed-peripheral electrolytic cell designed for stabilizing the foaam, the process may not apply for other kinds of workpieces.
The plasma process used for case hardening and coating is usually carried out in a low-pressure chamber. Because of the closed chamber, the process cannot be used to treat an article with a large surface or to treat a localized surface practically.
Plasma thermal spraying and plasma jet have been used for coating depositions in which plasma generated is based on gas medium. The plasma electrolytic carburizing/nitriding described in papers (X. Nie, et al, A Novel Modification Technique for Metal Surface, J.
of Wuhan University of Technology, (Materials Science Edition in English), vol 1 l, No. 1, pp. 28-35, Mar. 1996; X. Nie, et al, Characteristics of a Plasma Electrolytic Nitrocarburising Treatment for Stainless Steels, Surf. & Coat. Technol., vol.
139, pp.
135-142, 2001) is carried out by immersing workpieces in electrolytes. The capacity of power supply is required according to the surface area of the; workpiece to be treated.
Large woxkpieces are difficult to be treated because of requirement of a considerably large power supply and cooling system for the electrolyte cell.
The main drawback to the known methods of micro-arc (or micro-spark) oxidation (DE, A1, 4209733, U.S. Pat No. 5,385,662, RU, C1, 2070622) is the long time required to attain the sparking regimen, which in turn increase the duration of the entire coating formation process. The time of transition from the anodizing stage to the spark discharge, however, depends on the initial current density. To avoid the drawback above, the methods described in U.S. Pat. No. 5,275,713 and 6,365,028 attempt to begin the oxide coating process with a high current density. In all of the existing method using either AC, DC, or pulse DC power, however, the spark discharged region still attains for a quite long time (up to 30 seconds) before the spark discharge is extinguished and shifted onto new, colder areas of the surface, which causes porous and rough coatings of uneven thickness especially within the top layer of 30% total layer thickness (X. Nie, et al., Abrasive Wear/corrosion Properties and TEM Analysis of A1203 Coatings Fabricated Using Plasma Electrolysis, Surf. & Coat. Technol., vol 149, pp. 245-251, 2002; A.
Yerokhin, X.
Nie, et al., Review: Plasma Electrolysis for Surface Engineering, Surf. &
Coat. Technol., vol 122, pp. 73-93, 1999). Again, the power requirement for micro-arc oxidation depends on the area of workpiece to be treated; a large power supply is needed to treat large surfaces of a workpiece. For example, 160 kW is usually required to treat an aluminum sheet with a surface area of 0.1 square metre.

2 SUMMARY OF THE INVENTION
In the first aspect, the present invention relates to a process which is involved in plasma generated on or adjacent to the material surfaces in electrolytic or combinative electrolyte/gas/suspension environments established by arranging the materials to form either the cathode or anode of an electrolytic cell circuit in which the counter-electrode is the electrolytic jet head and the jetted electrolyte is the electrically conductive medium between the electrodes biased by a AC, preferably DC or pulse DC voltage in excess of 100 V.
In a second aspect, the present invention provides a method by which the electrolyte used is an aqueous solution impinged with air, Ar, NH3, NZ, O2, H2, or C02 gases into electrolytic flow andlor incorporated with metallic, ceramic, polymeric, composite powder additives into the solution at various percentages at a range of 0-50%
by volume of gas/solution and/or powder/solution, respectively.
In a third aspect, the present invention provides a process for cleaning and coating localized and large surfaces of conductive materials in an environmental-friendly and cost-effective manner by use of relatively small power supply.
In the fourth aspect, the present invention provides a process for case hardening (i.e., carburizing, carbonitriding, nitrocarburizing and nitriding) and coating (i.e., metallic, ceramic, polymeric and composite) of localized and large non-ferrous, or preferably ferrous metals and their alloys.
In the fifth aspect, the present invention provides a process iPor anodizing of localized and large surfaces of lightweight metals and alloys (e.g., Mg, Al and Ti), when the materials to be treated are used as the anode, in which the electrolytic jet provides interruption to the established plasma discharged region for reduction of plasma. duration time and thus a denser and smoother oxide coating can be produced.
BRIEF DESCRIPTION OF THE DRAWINGS
Illustrative and presently preferred embodiments of the invention are shown in the accompanying drawing in which:
FIG. 1 illustrates schematically the electrolytic jet/spraying plasma processing apparatus in a spraying mode according to the present invention.
FIG. 2 illustrates schemaxically the electrolytic jet/spraying plasma, processing apparatus in a jet mode according to the present invention.
FIG. 3 illustrates principally a sectional view in elevation of the electrolytic jet/spraying heads shown in FIG. 1 and FIG. 2 FIG. 4 illustrates principally a sectional view in elevation of the other electrolytic jetlspraying heads shown in FIG. 1 and FIG 2.
Referring to FIG. 1 of the drawings, an electrolytic jet/spraying head (1) is used to spray an electrolyte (2) with or without additives in forms of gas, liquid and powder onto the

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."~..._.._..._.-..__..-,-... _._ workpiece (3) to be treated. The workpiece (3) is negatively or positively biased by the power supply (4). The sprayed local surface of workpiece (3) generates plasma (S) by ionization of gas, which forms due to electrolysis and heating electrolyte or comes from additives added into the electrolyte flow, or by dielectric discharge, according to process parameters and characteristics of materials being treated. The electrolyte (2) is collected by the container (6) and flows back to the reservoir (7). The pump (8) and adjustable valve (9) supplies the pressurized electrolyte to the electrolytic jet/spraying head (1), composing a continuous operation.
Referring to FIG. 2 of the drawings, an electrolytic jetlspraying head (1) is used to jet an electrolyte (2) with additives of air or gas and with or without additives of liquid and powders into a container (6). The workpiece (3) to be treated is immersed into mixture of electrolyte and gas bulbs. The workpiece (3) is negatively o:r positively biased by the power supply (4). The sprayed local surface of workpiece (3) generates plasma (S) by ionization of gas, which forms due to electrolysis and heating electrolyte or comes from additives added into the electrolyte flow, or by dielectric discharge, according to process parameters and characteristics of materials being treaed. The electrolyte (2) is collected by the container (6) and flows back to the reservoir (7). The pump (8) and adjustable valve (9) supplies the pressurized electrolyte to the electrolytic jet/spraying head (1), composing a continuous operation.
Referring to FIG. 3 of the drawings, the typical electrolytic jet/spraying head (1) is composed of outer ( 10) and interior ( 11 ) tubes. The interior tube ( 11 a) with or without an electrical distribution grid (1 1b) has a number of holes (12) introducing air/gas flows provided from air/gas ring (13). The additives such as powders may also be added along with electrolyte flowing.
Referring to FIG. 4 of the drawings, the typical electrolytic jet/spaying head (1) has tubes ( 1 S, 16, 17, and 18) on it for addition of additives of gases, liquid reactant, and powders under pressures raging from 20 psi to 100 psi into the electrolyte flow.
The present invention will be further described with reference to the following examples:
Example 1 An electrolyte of 5-1S% sodium carbonate solution may be used for cleaning surfaces of contaminated or rusted ferrous and non-ferrous metals and their alloys which were passed in front of the electrolyte jetJspraying head shown in FIG. 1. The workpiece being treated was connected to cathode of the power supply with the bias voltage exceed of 80 V.
Example 2 A nitrogen-containing solution (such as NH3~H20 or urine fertilizer) may be used for nitriding or nitrocarburizing various metals and alloys which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG.

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2. The workpiece being treated was connected to cathode of the power supply with the bias voltage exceed of 80 V.
Example 3 An electrolyte of 5-10% sodium carbonate solution with NH3, N2, NZ/Ar or N2/C02 gas additive impinged into the electrolyte shown in FIG. 3 and FIG. 4 may be used for nitriding or nitrocarburizing various metals and alloys which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG.
2. The workpiece being treated was connected to cathode of the power supply with the bias voltage exceed of 80 V.
Example 4 An electrolyte of 5-10% sodium carbonate solution with additives of Zn-, Ni-, Al-, Cu-containing solution and ceramic powders impinged into the electrolyte shown in FIG. 3 and FIG. 4 may be used for coating various conductive material surfaces which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG. 2. The workpiece being treated was connected to cathode of the power supply with the bias voltage exceed of 80 V.
Example 5 An electrolyte of 5-15% passivating salt (such as silicate-, phosphate-, aluminate-, molybdate-, tungstate-based) solution with or without air additives impinged into the electrolyte shown in FIG. 3 and FIG. 4 may be used for oxidizing various lightweight materials (Al-, Mg-, and Ti- metals and their alloys) which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG.
2. The workpieces being treated such as piston, engine bore liner, engine box, transmission box, and other lightweight metallic/composite components were connected to anode of the power supply with the bias voltage exceed of 80 V.
Example 6 An electrolyte of 5-15% passivating salt (such as silicate-, phosphate-, aluminate-, molybdate-, and tungstate-based) solution with additives of lubricant powders (such as graphite, MoS2, BN, Teflon, et. al.) impinged into the electrolyte shown in FIG. 3 and FIG. 4 may be used for oxidizing various lightweight materials (Al-, Mg-, and Ti- metals and their alloys) which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG. 2. The workpieces being treated such as piston, engine bore liner, engine box, transmission box, and other lightweight metalliclcomposite components were connected to anode of the power supply with the bias voltage exceed of 80 V. The workpiece surfaces were deposited with composite coatings composed of oxides and lubricant particles.
~.~--Example 7 An electrolyte of 5-15% phosphate-based solution with additives of airs and bioceramic powders (such as HA, biodegradable powders, et. al.) impinged into the electrolyte shown in FIG. 3 and FIG. 4 may be used for coating various lightweight materials (Al-, Mg-, and Ti-metals and their alloys Which were passed in front of the electrolyte jet/spraying head shown in FIG. 1 or immersed in the solution shown in FIG. 2.
The workpieces being treated such as artificial hips and teeth and other bioimplants were connected to anode of the power supply with the bias voltage exceed of 8fl V.
The workpiece surfaces were deposited with composite coatings composed of oxides and bioceramic particles.

Claims

Claims What is claimed is:

1. A process involved in plasma generated on or adjacent to the material surface in electrolytic or combinative electrolyte/gas/suspension environments is established by arranging the materials to form the cathode or anode of an electrolytic cell circuit in which the counter-electrode is the electrolytic jet head and the jetted electrolyte is the electrically conductive medium between the electrodes biased by a AC, DC or pulse DC
voltage in excess of 80 V.

2. A process as claimed in claim 1, wherein the electrolyte used is an aqueous solution impinged with air, Ar, NH3, N2, O2, H2, or CO2 gases into electrolytic flow and/or incorporated with metallic, ceramic, polymeric, composite powder additives into the solution at various percentages at a range of 0-50% by volume of gas/solution and/or powder/solution, respectively.

3. A process as claimed in claim 1 or 2, wherein the process can be adapted for cleaning surfaces of contaminated or rusted ferrous and non-ferrous metals and their alloys which were passed in front of the electrolyte jet/spraying head.

4. A process as claimed in claim 1 or 2, wherein the process can be adapted for case hardening (i.e., carburizing, carbonitriding, nitrocarburizing and nitriding) various metals and alloys (including various cast steels and cast irons) which were passed in front of the electrolyte jet/spraying head.

5. A process as claimed in claim 1 or 2, wherein the process can be adapted for plasma deposition of coatings (i.e., metallic, ceramic, polymeric and composite) on localized and large conductive material surfaces which were passed in front of the electrolyte jet/spraying head.

6. A process as claimed in claim 1, 2, or 5, wherein the process can be adapted for plasma anodizing of lightweight metals and alloys (e.g., Mg, A1 and Ti) for wear and corrosion protection applications.

7. A process as claimed in claim 1, 2, ar S, wherein the process can be adapted for plasma deposition of oxide/lubricant composite coatings on lightweight metals and alloys (e.g., Mg, Al and Ti) for wear protection applications.

8. A process as claimed in claim 1, 2, or 5, wherein the process can be adapted for plasma deposition of bioceramic coatings on lightweight metals and alloys (e.g., Mg, Al and Ti) for bioimplant applications.